Upamanyu was a favorite disciple of Saint Ayodhdhaumya. He was assigned the task of looking after the cows. One day when he returned after his days work, the Guru called him and said, “Son, you look very healthy and radiant. What do you eat while you are working?” Upamanyu replied with all humility, “Sire, I eat whatever I get in alms.” The saint who wanted to test Upamanyu said that from now on he would not eat anything that he received as alms without first asking him. Upamanyu agreed and from then on offered all that he received to his guru. The Guru would take everything and give Upamanyu nothing.
A few days later, the Guru again asked Upamanyu, “I take away all that you beg for, so what do you eat?” Upamanyu replied that whatever he received by begging the first time, he offered it to his Guru and then went out and begged again a second time. The guru said, “Son, this conduct is inappropriate for a resident of the hermitage. By begging twice you hinder the earnings of other pupils as you cut into their share of alms. Besides this also proves your greed.”
Upamanyu listened to the guru and agreed not to do so again. A few days later, the guru again asked Upamanyu how he still looked so robust when he had stopped him from eating anything. To which Upamanyu replied that he drank a little of the cow’s milk while he took them out to graze. The guru forbade him to do that without his permission. A few days later, on seeing Upamanyu still looking healthy, the guru again called him and asked how he was surviving. This time Upamanyu said that he drank the foam that the calves drooled while feeding on their mother’s milk. The guru again forbade Upamanyu to do so and said that this would affect the health of the calves as they would drop more and more foam for Upamanyu to feed out of the kindness of their heart.
Upamanyu agreed to this too and went back to graze the cows as was his duty. Now that all avenues of getting food were closed, Upamanyu felt really hungry. When he could not tolerate the intensity of hunger he went and ate the leaves of a plant called `Aak’, the leaves of which produce a bitter, sour, acidic and poisonous juice. Due to the effect of this juice Upamanyu was instantly blinded. He kept wandering in the woods and fell into a dry well.
After sunset, when Uapmanyu did not return to the hermitage, the saint started worrying about him. He said to his other pupils, “I have closed all avenues for Upamanyu to get food. He is probably angry at this and hence has not returned, so let us all go and look for him.” So they went to the woods to look for Upamanyu. They called out for him and heard a feeble voice from the well. They hurry to the well and when they peeped inside they saw Upamanyu. They asked Upamanyu how he fell inside the well. Upamanyu said, “I was so hungry that I ate the leaves of the Aak plant. I then became blind and while I was trying to find my way home I fell into the the well.” After hearing his story they pulled him out of the well and his Guru told him to pray to the physicians of the Gods, Ashwini Kumar, so that his eyes would be healed. Upamanyu did as he was told and and Ashwini Kumars (twins who are the Physicians of Gods) appeared before him. They said, “Upamanyu, eat this sweet that we have brought you and you shall be healed.” Upamanyu declined and said that he cannot eat anything without the permission of his guru. The gods then told him that they had offered his guru some sweet and he ate it without asking the permission of his own Guru. So if it is okay for your Guru to eat without first asking permission from his own guru then it is okay for you too.
But Upamanyu again declined and this devotion and obedience of Upamanyu towards his guru pleased the gods. They blessed him and Upamnyu’s eyes were healed and all his teeth turned into gold. Upamanyu then went to his guru and was blessed by him. His guru was so pleased with him that he blessed him with instant memory and told him that he will know the Vedas and Dharamshashtras (other religious texts) automatically without any effort. ~ Mahabharata Paushya Parva SECTION III
This Upamanyu was dikshya guru of Sri Krishna. In Mahabharata Anushasan parva Book 13, chapters 15-16 explain that Krishna did severe penance for several months. Lord shiva,who was pleased with the devotion of Krishna and give him boons.
Devi Bhagavatha Purana states: Krishna thus approached the Great Sage Upamanyu, who was not only an expert of Vedas but a noted devotee of Lord Maha Deva with extraordinary mystic powers to initiate and guide Krishna in ascetic practice. Sri Krishna learnt from the Sage Upamanyu the Holy ‘Pasupati Mantra’.
Upamanyu was also narrated by Vaayudeva in Shiva Maha Purana.
~ Sri Krishna once came to meet the sage Upamanyu looking for a guru where Upamanyu states he is the son of hermit named Vyaghrapada in krtayuga and Dhaumya is his brother.
Upamanyu taught Krishna the words of wisdom that he learned from Shiva; he also taught Krishna to pray to Shiva. It was by praying to Shiva that Krishna obtained his son Samba. For sixteen months Krishna had to pray before Shiva appeared, to grant the boon regarding the son. Parvati also granted Krishna several boons.